The present invention relates to a skew compensation device for use in helical-scan magnetic video tape playback or reproduction apparatus and the method of using that skew compensation device, and more particularly, to a device and a method for compensating for a skewing phenomenon generated when performing special reproduction operations, such as fast forward search, in a long-play (LP) mode.
Generally speaking, helical-scan magnetic video tape playback apparatus (for example, a videocassette player or a videocassette recorder/player) operating based on the NTSC system can utilize a standard-play (SP) mode, an LP mode and a super-long-play (SLP) mode, but helical-scan video tape playback apparatus operating based on the PAL system or the SECAM system can only utilize the SP and LP modes.
FIGS. 1A, 1B and 1C show the various tape formats used in connection with the above SP, LP and SLP modes, respectively. 1 H is the duration of a video scan line as recorded along a track on the video tape. FIGS. 1A and 1C illustrate the fact that an alignment of horizontal scan lines (or "H-alignment") on adjoining tracks results when recording in the SP mode or the SLP mode. Horizontal scan lines exhibit similar spatial phasing on parallel recording tracks, both adjoining ones and alternate ones. FIG. 1B illustrates the fact that horizontal scan lines exhibit staggered spatial phasing along adjoining tracks and along alternate tracks when recording in the LP mode. Horizontal scan lines exhibit similar spatial phasing only every fourth one of parallel recording tracks when recording in the LP mode, as illustrated in FIG. 1B. Accordingly, no skewing phenomenon associated with irregularity in horizontal scan line phasing from one recording track to the next is encountered in the SP or SLP mode special reproducing functions, such as rapid search. However, the skewing phenomenon manifests itself in the LP mode special reproducing functions, such as rapid search or quick find, since horizontal scan line phasing varies between recording tracks by as much as 0.5 H. A device for compensating for this skewing is called a "skew jump device" or "skew compensation device".
In other words, based on the tape format shown in FIG. 1B, when a search operation is performed in the LP mode with two long-play heads, so that each head jumps a closest-by track the azimuth of which differs from that of the current track and then performs a reproducing operation on the next-to-closest-by track, which presumably has the same azimuth, a 0.5 H delay mismatch generally occurs and the reproduced image is distorted accordingly. Therefore, a skew compensation device is required for compensating for such a delay mismatch in the tracking.
The circuit diagram of a conventional skew compensation device including skew jump detector 60 for generating a skew jump indication signal, a 0.5 H delay line 70, and an output video signal selection switch 80 is shown in FIG. 2, together with other apparatus used during special reproducing functions, such as rapid search. Four head signals (SP+, SP-, LP+ and LP-) are picked up by a head unit 10. A preamplifier 20 preamplifies these head signals in preamplifiers 21, 22, 23 and 24, respectively. In response to a head switching signal and a SP/LP head selection signal supplied thereto from a microprocessor 30, the preamplifier 20 selects the response of one of the preamplifiers 21, 22, 23 and 24 as its output signal supplied to a video reproducer 40 as its input signal.
The microprocessor 30 generates the head switching signal in accordance with control pulses derived from a crystal oscillator in chroma reproduction circuitry. The microprocessor 30 generates the SP/LP head selection signal in accordance with an envelope comparison signal supplied from the preamplifier 20 and playback mode indications as to whether the video tape being played back was recorded in SP, LP or SLP mode and as to whether playback is at normal-reproduction tape speed or at special-reproduction tape speed. The playback mode indications can comprise a normal/special signal supplied in response to user control, which indicates whether playback is at the speed associated with normal reproduction or at a multiple of that speed associated with special reproduction, and tape speed indications of whether the video tape being played back was recorded in SP, LP or SLP mode supplied from automatic speed selection circuitry. Where the automatic speed selection function is performed within the microprocessor, rather than in automatic speed selection circuitry completely external to the microprocessor, the microprocessor is supplied pulses that a control head senses from the control track on the video tape.
In a PAL and SECAM type of videocassette recorder, a color phase error occurs when in the LP mode, so that proper color representation is not made; i.e., an erroneous color is represented on a television monitor. In a PAL-type system, the phases of a burst signal and a color carrier signal are inverted every 1 H period. In the a SECAM-type system, the R-Y and B-Y color signals are frequency-modulated by 4.4 MHz and 4.25 MHz, respectively, and the carrier signal of a chroma FM signal is switched between 4.25 MHz and 4.4 MHz every other 1 H period, so color representation is not stably performed in the LP search mode. Therefore, a reproduced color signal muting unit 50 is customarily used to mute or "kill" the PAL-type or SECAM-type color signals, to avoid presenting an erroneous color image. However, in an NTSC type of videocassette recorder, there is no switching of color subcarrier phasing between lines, so the reproduced color signal muting unit 50 is not used and the output signal of the preamplifier 20 is supplied to the video reproducer 40 without muting the color signal. Instead, a color signal compensating unit for compensating for a phase error generated during the LP mode, can be provided at the output of the video reproducer 40.
FIG. 3 is a position map of the four heads situated on a drum in the head unit of a helical tape recorder. When special reproducing functions are performed in the SP mode, all four heads are utilized. However, when special reproducing functions are performed in the LP mode or in the SLP mode, only two heads (those supplying the LP+ and LP- head signals) are utilized. Referring to FIG. 2, four head signals (SP+, SP-, LP+ and LP-) are induced in respective windings of the heads and the preamplifier 20 supplies preamplified response to one of them as the input signal for the video reproducer 40.
First, the operation of the preamplifier 20 when triple-speed searching in the SP mode will be explained. Head selection is made in accordance with the head switching signal (HEAD SW) which is shown in FIG. 4A. When the head switching signal is at a logic "low" level, first and second head selecting switches 25 and 26 select the signals which are picked up by the SP- head and LP+ head and transferred through second and fourth amplifiers 22 and 24. When the head switching signal is at a logic "high" level, the first and second head selecting switches 25 and 26 select the signals which are picked up by the SP+ head and LP- head and transferred through first and third amplifiers 21 and 23.
In an envelope comparator 27, a comparison is made between the SP- head signal and the LP+ head signal or between the SP+ head signal and the LP- head signal, according to the selection made by the first and second head selecting switches 25 and 26. The envelope comparator 27 includes envelope detectors for each of its input signals, means for differentially combining the detection results, and means for determining the polarity of the result of differentially combining to generate the envelope comparison signal. If the LP head signal (LP- or LP+) is larger than the SP head signal (SP- or SP+), the envelope comparator 27 supplies a logic "high" level of envelope comparison signal (ENV. COMP) to the microprocessor 30. If the LP head signal is smaller than the SP head signal, the envelope comparator 27 supplies a logic "low" level of envelope comparison signal to the microprocessor 30. The envelope comparison signal output from the envelope comparator 27 is shown in FIG. 4B.
The microprocessor 30 performs pulse shaping on that envelope comparison signal to produce a signal shown in FIG. 4C for controlling a head amplifier switch 28 that selects either the LP head signal or the SP head signal as the preamplifier 20 output signal. In other words, when the SP/LP head selection signal produced from the microprocessor 30 is "high", the head amplifier switch 28 selects the LP head signal output from second head selecting switch 26 as the preamplifier 20 output signal; and when the SP/LP head selection signal is "low", the head amplifier switch 28 selects the SP head signal output from the first head selecting switch 25 as the preamplifier 20 output signal. The preamplifier 20 output signal so selected is supplied to the video reproducer 40 as its input signal.
Now, the operation of the preamplifier 20 when triple-speed searching in the LP mode or in the SLP mode will be explained. In these search modes, the head switching signal supplied from the microprocessor 30 to the head selecting switches 25 and 26 is as shown in FIG. 4D, and the envelope comparison signal that the envelope comparator 27 supplies to the microprocessor 30 is as shown in FIG. 4E. In the LP and SLP modes, searching is made using heads LP- and LP+, so the SP/LP head selection signal output from the microprocessor 30 is always high, and the head amplifier switch 28 selects only the signals picked up by LP- and LP+ heads and transferred through the second head selecting switch 26 in accordance with the SP/LP head selection signal. The signal selected by the head amplifier switch 28 is reproduced by the video reproducer 40.
The operation of the FIG. 2 skew compensation device comprising the skew jump detector 60, the 0.5 H delay line 70, and the output video signal selection switch 80 will now be described. When special reproduction is performed in the SP mode or in the SLP mode, in which modes skew generation does not manifest itself, the skew jump detector 60 is conditioned to be inoperative. The control signal supplied to the switch 80 is in a "high" state, conditioning the switch 80 to select the undelayed output signal of the video reproducer 40 as a selected video output signal. When special reproduction is performed in the LP mode now the switch 80 responds to a skew jump indication signal (SKEW JUMP) output from the skew jump detector 60 for selecting either the undelayed output signal of the video reproducer 40 or that output as delayed one-half scan line by the delay line 70, to produce a selected video output signal.
While the first and second head selecting switches 25 and 26, the head amplifier switch 28 and the output video signal selection switch 80 are shown as simple single-pole/double-throw switches those familiar with the art of video recording will appreciate that each of these switches is an electrically controlled electronic switch, as can be constructed in a variety of known ways recognized by those skilled in the art as essentially being equivalents. Head switching can be done by alternately powering the preamplifiers 21, 22 and by alternately powering the preamplifiers 23, 24, for example.
The operation of the FIG. 2 skew compensation device is more particularly described with reference to the more detailed FIG. 5, in which diodes D1 and D2 provide the means for selectively conditioning the skew jump detector 60 to be operative to selectively switch the switch 80 in the LP mode, but not in the SP or SLP modes. As long as the diode D1 remains reverse-biased for non-conduction, the switch 80 responds to a skew jump indication signal (SKEW JUMP) supplied from the skew jump detector 60 via the forward-biased diode D2, for selecting either the undelayed output signal of the video reproducer 40 or that output as delayed one-half scan line by the delay line 70, to produce a selected video output signal. The SKEW JUMP signal is supplied from the skew jump detector 60 to the switch 80 via the diode D2 only as long as it remains forward-biased for conduction, which the diode D2 does as long as diode D1 remains reverse-biased for non-conduction. A pull-down resistor (not shown in FIG. 5) connected from the interconnected cathodes of the diodes D1 and D2 to a point of "low" potential will aid in maintaining the desired bias conditions on the diodes D1 and D2.
When special reproduction is performed in the SP mode or in the SLP mode, in which modes skew generation does not manifest itself, the LP search signal is "high". This forward-biases the diode D1 for conduction and thereby applies a "high" level of control signal to control switch 80, conditioning the control switch 80 to select the undelayed reproduced signal of the video reproducer 40 as the selected video output signal. The diode D2 is reverse-biased for non-conduction during the times the skew jump indication signal is in a "low" state, so the skew jump indication signal being in a "low" state cannot condition the control switch 80 to select the delayed reproduced signal of the video reproducer 40 from the delay line 70 as the selected video output signal.
When special reproduction is performed in the LP mode, the LP search signal is "low", so the diode D1 is not forward-biased into conduction. Accordingly, the skew jump indication signal can forward-bias the diode D2 into conduction when it is in a "high" state. If the skew jump indication signal is in a "high" state, the output signal of the video reproducer 40 is selected as the selected output signal; and if the skew jump indication signal is in a "low" state, the output signal of the video reproducer 40 as delayed by one-half scan line by the 0.5 H delay line 70 is selected as the selected video output signal.
FIG. 5 shows the skew jump detector 60 in greater detail than FIG. 2 as comprising the diodes D1 and D2, a sync signal separator 61, a phase comparator 62, a voltage-controlled oscillator 63, a frequency divider 64, and a data or D flip-flop 65. The sync signal separator 61 separates a horizontal sync signal per FIG. 6A from the video signal produced by the video reproducer 40 of FIG. 2. The phase comparator 62 compares the phase of the horizontal sync signal separated by the horizontal sync separator 61 with that of the output signal of the frequency divider 64. The voltage-controlled oscillator 63 receives a phase difference signal from the phase comparator 62 as an automatic frequency and phase control voltage, which regulates the oscillation of the voltage-controlled oscillator 63 so that it oscillates at the frequency corresponding to 2f.sub.H thus producing a signal as shown in FIG. 6B. The frequency divider 64 halves the frequency of the output signal of voltage-controlled oscillator 63, to produce a signal as shown in FIG. 6C. The output signal of the frequency divider 64 will be referred to as horizontal scanning frequency signal f.sub.H in the remainder of this disclosure.
The D flip-flop 65 receives the signal output from the frequency divider 64 and receives the horizontal sync signal output from the horizontal sync separator 61 as a clock signal, to sense the portion in which a skew occurs and to produce the skew jump indication signal as shown in FIG. 6D. In the case where horizontal scanning frequency signal (shown in FIG. 6C) is high when horizontal sync signal (shown in FIG. 6A) is high, a skew jump indication signal having a "high" level is produced by the data flip-flop 65. When the diode D2 is forward-biased for conduction, during the LP mode, the switch 80 responds to the "high" level of skew jump indication signal to select the original video signal output from the video reproducer 40 as its selected output video signal. On the other hand, in the case where the horizontal scanning frequency signal (shown in FIG. 6C) is low when the horizontal sync signal (shown in FIG. 6A) is high, which case corresponds to when 0.5 H skew jump occurs, the skew jump indication signal (shown in FIG. 6D) should switch from "high" to "low" state. If such is the case, during the LP mode when the diode D2 is forward-biased for conduction, switch 80 responds to the "low" level of skew jump indication signal to select the 0.5 H delayed video signal output from the 0.5 H delay line 70 as its selected output video signal.
The phase of horizontal scanning frequency signal f.sub.H from the frequency divider 64 is very important for properly sensing when skew jumps occur, so the phasing thereof must be precisely controlled. If the phase slips from proper phasing, a skew jump erroneously occurs, wrecking the image recovered from the selected video output signal provided by the switch 80. Since the automatic frequency and phase control voltage of the voltage-controlled oscillator 63 is generated in response to horizontal sync signal reproduced from the video tape and separated by the horizontal sync separator 61, there is a problem with the phase of the horizontal scanning frequency signal f.sub.H accurately tracking the effects of variations in tape motion past the reproducing heads on the constancy of the phasing of the separated horizontal sync pulses. Inaccurate tracking has adverse affects on the image because the erroneous occurrences of skew jump are more frequent. Also, a separate circuit as that shown in FIG. 5 for generating a skew jump indication signal complicates the circuit configuration unduly, the inventor has discerned.
In U.S. Pat. No. 5,245,482 issued 14 Sep. 1993 and entitled Magnetic Recording/Reproducing Apparatus with Skew Correction, Sagawa et alii describe an alternative device for compensating a video signal skew generated when scanning a magnetic tape using a plurality of heads in a magnetic recording/reproducing apparatus such as videocassette recorder. This device performs a head switching during vertical blanking period and includes circuitry to prevent jitter in a reproduced video image causing a skew jump to occur during head switching. However, this prior-art device also requires a separate circuit for compensating for skewing, which has an additional cost associated therewith.